1. Field of the Invention
The present invention relates to an optical fiber preform, an optical fiber, and a method of manufacturing an optical fiber preform.
2. Description of the Related Art
An optical fiber composed of silica-based glass and having a core region doped with an alkali metal is known (refer to Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2007-504080 and U.S. Patent Application Publication No. 2006/0130530). In the case where a core portion of an optical fiber preform is doped with an alkali metal, it is possible to reduce the viscosity of the core portion during drawing of the optical fiber preform, as compared with the case where the core portion is not doped with an alkali metal. Thus, relaxation of a network structure of silica glass can be made to enhance. It is believed that, consequently, an attenuation of the resulting optical fiber manufactured by drawing can be reduced.
A known method of doping silica glass with an alkali metal is a diffusion method. In the diffusion method, a glass pipe composed of silica-based glass is heated by an external heat source or non-equilibrium plasma is generated in the glass pipe while vapor of a raw material such as an alkali metal or an alkali metal salt is introduced into the glass pipe, thereby doping an inner surface of the glass pipe with an alkali metal element by diffusion.
After the glass pipe is doped with an alkali metal by the diffusion method, the glass pipe is heated to reduce the diameter thereof. After the reduction of the diameter, in order to remove impurity, such as Ni, Fe, and other transition metals, which were also doped to the glass pipe at the same time as the doping of the alkali metal element, the inner surface of the glass pipe is etched by a certain amount in the direction of the wall thickness. The diffusion rates of alkali metals are higher than those of transition metals. Therefore, even when the transition metals are removed by etching a surface of glass by a certain amount in the thickness direction thereof, the most of doped alkali metals can remain in the glass. After the etching, the glass pipe is made to collapse by heating to manufacture an alkali-metal-doped core rod. A cladding portion is synthesized on the alkali-metal-doped core rod to manufacture an optical fiber preform. Subsequently, the optical fiber preform is drawn to manufacture an optical fiber.